2.
Pulls all the answers ids using the fastest/lightest API method (questions/id/answers, pagesize=100, body=false..), get the index of the accepted_answer_id on this list and finally find the page dividing accepted_answer_id position by the pagesize

Suppose that the answer id is passed to a web app via a URL. You want to show the page of answers that has that answer answer. How can that be done?
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Nathan Osman♦Jun 22 '10 at 21:45

@George Edison - hmm, if the sort if votes it can't be done; since answers with the same score are randomly ordered there is no canonical page. If its anything else, binary search w.r.t pages would be fastest. I still think its a pretty arbitrary thing to do.
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Kevin Montrose♦Jun 23 '10 at 1:12

It's for my Mobile Site if that helps. I get URLS like http://...../questions/xxx#yyy where yyy is the answer number. This way I have a named anchor on the page that the browser can jump to.
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Nathan Osman♦Jun 23 '10 at 1:31